1. Field of the Invention
The present invention concerns a method of assigning one or more spreading sequences to users of a Multi-Carrier transmission network, such as a Multi-Carrier Code Division Multiple Access transmission network, better known by the name MC-CDMA network or an Orthogonal Frequency and Code Division Multiplexing transmission network, better known by the name OFCDM.
1. Discussion Of The Background
MC-CDMA has been receiving widespread interest for wireless broadband multimedia applications. Multi-Carrier Code Division Multiple Access (MC-CDMA) combines OFDM (Orthogonal Frequency Division Multiplex) modulation and the CDMA multiple access technique. This multiple access technique was proposed for the first time by N. Yee et al. in the article entitled “Multicarrier CDMA in indoor wireless radio networks” which appeared in Proceedings of PIMRC'93, Vol. 1, pages 109-113, 1993. The developments of this technique were reviewed by S. Hara et al. in the article entitled “Overview of Multicarrier CDMA” published in IEEE Communication Magazine, pages 126-133, December 1997.
Unlike DS-CDMA (Direct Sequence Code Division Multiple Access), in which the signal of each mobile terminal or user is multiplied in the time domain in order to spread its frequency spectrum, the signature here multiplies the signal in the frequency domain, each element of the signature multiplying the signal of a different sub-carrier.
In general, MC-CDMA combines the advantageous features of CDMA and OFDM, i.e. high spectral efficiency, multiple access capabilities, robustness in presence of frequency selective channels, high flexibility, narrow-band interference rejection, simple one-tap equalisation, etc.
A MC-CDMA base station transmitter transmits a plurality of symbols to a plurality K of users or more precisely to the mobile terminal of users. For example, a MC-CDMA transmitter located in a base station of a MC-CDMA transmission system transmits symbols to a plurality of users over a plurality of downlink transmission channels.
A complex symbol to be transmitted from the base station to user k is first multiplied by a spreading sequence denoted ck. The spreading sequence consists of L “chips”, each “chip” being of duration Tc, the total duration of the spreading sequence corresponding to a symbol period T. In order to mitigate intra-cell interference, the spreading sequences are chosen orthogonal.
The result of the multiplication of the complex symbol by the elements of the spreading sequence for user k gives L complex values that are added to the similar values to be transmitted to the other users k′≠k. These values are then demultiplexed over a subset of L frequencies of an OFDM multiplex, then subjected to an Inverse Fast Fourier Transformation (IFFT). In order to prevent intersymbol interference, a guard interval of length typically greater than the duration of the impulse response of the transmission channel, is inserted in front of to the symbol outputted by the IFFT module. This is achieved in practice by adding a prefix (denoted Δ) identical to the end of the said symbol. The resulting symbol is then filtered and transmitted by the base station to a plurality of users.
The MC-CDMA method can essentially be regarded as a spreading in the spectral domain (before IFFT) followed by an OFDM modulation.
It is known that the propagation channel can be obstructed by houses and other obstacles situated between the transmitter and the receiver. The transmitted signal is then propagated on multiple paths, each path being delayed and attenuated differently. It should be understood that the propagation channel then acts as a filter whose transfer function varies with time.
The ability of MC-CDMA transmission networks to provide orthogonality between the signals of the different users in the network (and therefore to prevent any interference between these signals) depends on the intercorrelation properties of the spreading sequences which are assigned to the users of the network.
Typically, in the case of transmissions on a mobile radio channel from a base station to a set of mobile stations called hereinafter users or active users, the signals intended for each user are transmitted synchronously. Under these conditions, Walsh-Hadamard spreading sequences can be used to guarantee orthogonality between the users if the channel is not frequency selective.
In the European patent EP 1085689 it is disclosed a method of assigning one or more spreading sequences to a user of a MC-CDMA transmission network, wherein a spreading sequence is assigned to a user taking into account a predetermined set of spreading sequences. More precisely, the predetermined set of spreading sequences consists in the spreading sequences which minimize a function representing the interference between the spreading sequence and the spreading sequences of the said predetermined or given set.
Such method which makes it possible to reduce the effects of the interference on the performance of the transmission network under consideration needs long calculation in order to define the predetermined set of spreading sequences. Furthermore, such method doesn't take into account new technique such as adaptive antenna arrays also referred to as intelligent antennas or smart antennas.
The two main factors that make mobile radio reception difficult is the presence of multipath fading and co-channel interference. The use of smart antennas can improve performance in these hostile environments. Smart antennas can be broadly classified into two categories, namely the phased array and the diversity array.
A phased array consists of a set of antenna elements that are spatially distributed at known locations with reference to a common fixed point. By changing the phase and amplitude of the exciting currents in each of the antenna elements it is possible to create gains and nulls in any direction. The signals received in these elements are typically combined at baseband using complex weights. Adaptive algorithms can be used to adapt the weights based on some optimisation criteria such as maximization of output Signal to Noise Ratio. In such a system the antenna response is maximised in the direction of the desired user and minimized it in the direction of the interferer. The radiation pattern of the array is determined by the radiation patterns of the individual elements, their orientation in space and the amplitude and phase of the feeding currents.
Beamforming is the process of forming beams towards the direction of the desired user while simultaneously suppressing signals originating from other directions.